The present invention relates to an electrophoretic process for separating aqueous suspensions of fine mineral solids into solid and liquid components.
One problematic aqueous suspension of fine mineral solids arises as a waste tailings stream from commercial oil sand extraction plants. Two such extraction plants are presently mining and extracting the oil sands of the Athabasca oil sand deposit of Alberta, Canada, this being one of the largest oil sand deposits in the world. The oil sands exist as sand beds partially saturated with viscous bitumen. The bitumen content in the deposits ranges from less than 1% to about 18%. The grains of sand in these deposits are enveloped with a film of hygroscopic water containing fine mineral solids of silt and clay. The bitumen adheres as a film over this water layer. To extract the bitumen from the sands, a process known as the hot water extraction process is used. Briefly, the process includes mixing steam and hot water with the mined oil sands to form a dense pulp, then adding further quantities of hot water and practising flotation to separate the sand from the bitumen. The bitumen is skimmed off as a froth product and further upgraded, while the remainder of the water and solids are rejected as tailings streams. Process aids, including alkali and surfactants are added in the above steps to improve the bitumen separation. The majority of these process aids, together with unrecovered bitumen, are lost with the tailings stream. This tailings is produced at a rate of about 1 U.S. Gal./min. for every 10 Bbl./day of bitumen produced.
The sand solids settle rapidly from this tailings stream and clear water accumulates on the top leaving a stream, termed tailings sludge, which consists of a thixotropic suspension of water, the fine mineral solids released from the sand grains during the extraction process, unrecovered bitumen and lost process aids. The mineral content of this sludge ranges from about 11% to 40% by weight. The tailings sludge formes a relatively stable, non-settling suspension which must be discarded.
Presently, the tailings streams are disposed of in long term settling ponds. The tailings accumulate in these ponds at a rate of about 20,000 acre feet/year for a 100,000 Bbl./day oil sand extraction plant. These settling ponds, which are highly viscous and contaminated with bitumen, pose serious environmental hazards. The ponds also represent a significant loss of valuable process water and chemical process aids, and present a serious obstruction to the future recovery of the underlying bitumen deposits.
A number of different techniques have been proposed to date to cope with the growing problem of tailings disposal. Such techniques include filtration, chemical and biochemical flocculation, and centrifugation. The objective is always the same, to separate the suspended mineral fines from the aqueous phase to create a compact disposable solid product and a clarified, reusable water product. In each instance, however, the energy or chemical costs have been high and drying of the solid mineral phase to a water content of less than 60% has been difficult, if not impossible. It is usually desirable to obtain a solids product of greater than 40% solids and a clarified liquid product of less than 4% solids. These limits, while somewhat arbitrary, tend to define a solids product which is sufficiently dry for disposal, and a liquid product which can be reused as process water in the hot water extraction process.
The problem in obtaining a sufficiently dry deposit arises from the fact that a substantial portion of the sludge water is, through chemisorption, intimately associated with the clay particle structure and is essentially immobilized. The influence of normal mechanical forces does not appear to be able to dislodge this water from the clay.
Electrophoresis and electroosmosis are techniques which have been investigated in separating various types of solids from liquid-solid suspensions. Electrophoresis involves applying an electrical potential between electrodes in a liquid-solid suspension to cause the solids to migrate toward one of the charged electrodes. Electroosmosis, often termed reverse osmosis, involves applying an electrical potential in a liquid medium, particularly water, to cause the liquid to move, usually through a permeable barrier, toward a charged electrode. In aqueous suspensions of fine mineral solids such as clay, electrophoresis causes the mineral solids to migrate toward and deposit on the anode, whereas electroosmosis causes the water to migrate out of the clay deposit.
Various schemes for electrophoretically separating solids from liquid-solid suspensions are disclosed in U.S. Pat. Nos. 3,980,547 and 4,003,811, issued to A. C. Kunkle, 3,962,069 issued to K. Inoue et al., and 4,207,158 issued to M. P. Freeman. While these schemes may be effective in causing solids to deposit on an electrode surface, from my experience, when applied to an aqueous suspension of mineral fines, they would not provide a solids deposit sufficiently reduced in water content, for disposal.
In fact, in a study done by B. C. Flintoff and L. R. Plitt, published in Canadian Metallurgical Quarterly, 15, (1976), pg. 235-242, the conclusion was reached that only a limited degree of clarification could be achieved by electrophoretic separation of such aqueous suspensions of mineral solids as bentonite and oil sand tailings.